A protection relay, also referred to as an Intelligent Electronic Device (IED), is a microcontroller based intelligent electronic device with a basic function to protect electrical equipment by tripping a circuit breaker and interrupting a power line in case of over current or earth fault situations. The tripping signal on behalf of a trip coil or other actuator of the circuit breaker is generated by the protection relay, such as when the measured current in the line exceeds a nominal or preset value for a predefined time period. In certain situations such as Ring Main Unit (RMU) installations in urban areas, a self-supplied relay may, for example, be used. The self-supplied protection relay utilizes energy from the current sensing transformers to supply to the relay electronics circuit and the energy required to operate trip coils. The design of a self-supplied relay can have several constraints associated with it to ensure the measurements are accurate and sensitive. Also, special provisions can be included to make its circuitry efficient and optimized for power consumption. Some of these constraints and methods to generate power supply by controlled charging are disclosed in the WIPO publication WO 2009101463.
Inrush (e.g., current surge) can be observed during switching in large inductive loads such as a transformer, or an induction motor. In the field, the protection personnel set the earth fault settings on a higher side (or even double) so as to avoid any mal-operation because of inrush at load start up. They then reduce the settings to lower values once the load is under normal operation condition. This is only possible where point load (e.g., a single load like a motor or transformer) is being protected. The same may not be possible when a feeder is being protected. There could be different loads being switched in and out of the feeder and hence there could be multiple possibilities of inrush presence.
The exemplary values for 1 nominal (In) are 1A and 5A. An exemplary nominal current for illustration purposes for the relay described herein is taken as 1A. The exemplary range is 0.1 In to 20 In for an earth setting with an auxiliary powered relay. Even in such relays, it can be difficult to set relays for 0.1 In earth setting in field if they are not having inrush blocking protection. At places where inrush is evident and if relays are not having inrush protection and if the relay is set for the lower earth fault settings (e.g., of the order of 0.1 In), the relay generates a trip imagining an earth fault. This is however, not a true earth fault as an inrush is not a fault condition.
In such cases, it could be possible that a sensitive (lower) earth fault setting like 0.1 In is not used. The same is applicable to phase protection also. As a result, there are relays with a feature of automatic setting double the value for a limited time to avoid mal operation because of inrush.
Further, achieving the functionality for a self powered relay can be complicated. The self powered relays can involve minimum time and phase currents to get powered on. Also, the inrush detection can involve some additional time. With all these constraints it is desirable that a relay shall give trip within a desired minimum trip time if there is a genuine fault.
Further, the design of the relay should account for good current detection sensitivity. The sensitivity, here, refers to an ability to, for example, sense a minimum of earth fault leakage currents. Better sensitivity can help to identify possible major faults earlier.
High end or point load protection relays may have inrush protection as a built-in feature. However, implementation of inrush protection in the case of a self powered relay with sensitive earth fault protection would be a unique feature.
Accordingly, the present disclosure is directed to an efficient self-powered three phase non-directional overcurrent and earth-fault protection TED.